Penetration Depth of a Plunging Jet: From Microjets to Cascades.

When a liquid jet impacts a pool of the same liquid, a bubble cloud is generated below the surface: we address here the question of the vertical size of this bubble cloud. We have measured this size on experiments spanning a large range of scales, from 0.3 to 210 mm in jet diameter, and from 0.2 to 9.5 m in height of fall. We show that a simple model based on a balance of forces at the scale of the bubble cloud is able to account for all experimental results over this wide range of scales and types of jets. We derive from this model a simple expression for the prediction of the bubble cloud penetration depth.

Influence of Gas Turbulence on the Instability of an Air-Water Mixing Layer.

We present the first evidence of the direct influence of gas turbulence on the shear instability of a planar air-water mixing layer. We show with two different experiments that increasing the level of velocity fluctuations in the gas phase continuously increases the frequency of the instability, up to a doubling of frequency for the largest turbulence intensity investigated. A modified spatiotemporal stability analysis taking turbulence into account via a simple Reynolds stress closure provides the right trend and magnitude for this effect.

Induced agitation in homogeneous bubbly flows at moderate particle Reynolds number.

We investigated the agitation induced in a liquid by quasimonodispersed spherical bubbles. For Re(p) about 10 and for void fractions ranging from 0.002 to 0.1, the axial velocity fluctuation probability density functions happen to be uniquely determined by the velocity variance. In addition, the variance experiences a rupture from a nonlinear to a quasilinear increase with the volumic concentration. It is shown that the presence of a deficit of neighbors in the wake of any test inclusion forces the screening of velocity disturbances at a finite distance X-of order Re(p)/alpha-and that the velocity variance does not depend on the system size when the later is large compared with X. Finally, the bubble-induced agitation happens to be much stronger for quasimonodispersed than for polydispersed size distributions because of the longer interaction duration between pairs in the former case.

Turbulent transport of material particles: an experimental study of finite size effects.

We present experimental Lagrangian statistics of finite sized, neutrally bouyant, particles transported in an isotropic turbulent flow. The particle's diameter is varied over turbulent inertial scales. Finite size effects are shown not to be trivially related to velocity intermittency. The global shape of the particle's acceleration probability density functions is not found to depend significantly on its size while the particle's acceleration variance decreases as it becomes larger in quantitative agreement with the classical k(-7/3) scaling for the spectrum of Eulerian pressure fluctuations in the carrier flow.